Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

Chapter 2 Findings in Doel 3 and Tihange 2

2.1 Introduction

According to the ASME Code Section XI, a 10-yearly examination of the RPV must be done. The ASME Code requires a surface examination by Visual Testing (VT) of the complete inner side of the RPV and a volumetric examination by Ultrasonic Testing (UT) of the weld lines between different rings of the RPV and the Heat Affected Zones (HAZ) around the welds. [8] The areas to be examined are indicated in Figure 2.1. Back in February 1999, an extra volumetric examination by UT was performed on the whole core shell, also called the beltline, of the RVP of the PWR of Tricastin 1 in France. This inspection resulted in the detection of some underclad defects. [10] These type of defects are formed during the cladding process. As the cladding is weld-deposited on the base material, the cooling after deposition will result in thermal stresses. These stresses can overcome the yield stress of the base material and result in cracks. The formed cracks are oriented perpendicular to the cladding- base material interface (Figure 2.2). [11] Due to their orientation, they will be subjected to both hoop and axial stresses (Figure 2.3). Temperature and pressure gradients over the RPV wall can result in significant stresses in the hoop and axial directions. [9] Therefore, the existence of underclad defects are a particular concern for the mechanical integrity of the RPV. As a result of the discovered underclad defects in Tricastin 1 and its possible detrimental consequences for the integrity of the RPV, Electrabel, the operator of the Belgian NPPs, performed UT inspections to find such underclad defects in the beltline of the Doel 3 RPV during the planned outage in June 2012. These UT inspections examined the first 30 mm from the inner pressure vessel surface, additional to the UT inspections according to the ASME Code mentioned above. [1] 11